Titanium has outstanding resistance to corrosion, and is extensively used as an industrial material. For example, titanium is used in plants for the manufacture of chemicals, for power generation, and for the desalination of seawater. The high specific strength of titanium also makes it useful as a construction material for aircraft and the like.
The disadvantage of titanium is that it is prone to embrittlement with hydrogen absorption. When hydrogen molecules or atoms are present on its surface portion, titanium will readily absorb hydrogen to transform the titanium material into a brittle titanium hydride. As the amount of hydride formed increases, the titanium material becomes increasingly embrittled, which eventually causes it to rupture upon subjection to but a slight force. The environments that allow titanium to absorb hydrogen are, e.g., power plants where turbine blades are exposed to high-temperature steam or hot hydrogen gas.
Hydrogen absorption by titanium takes place in aqueous solutions too. This is attributed to the fact that, in the case of cathodic protection, galvanic corrosion, or the corrosion of titanium itself, a cathodic reaction 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 occurs on the titanium surface. The cathodic reaction causes a part of the resulting hydrogen to be taken up by titanium. Abundant literature exists on hydrogen absorption by titanium in such aqueous solutions. Methods already known for preventing it include, e.g., forming a thick titanium oxide film or a titanium nitride layer on the titanium surface.
A method of producing a titanium material for the purpose of providing excellent corrosion resistance in a aqueous acidic solution is described in U.S. Pat. No. 4,908,072 issued Mar. 13, 1990 to Taki, et al. A layer of at least one of titanium nitride, titanium carbide or titanium carbonitride is formed on the surface of a titanium material to provide corrosion resistance. The coated titanium materials were tested in an aqueous hydrogen chloride solution. Taki, et al. concluded that such titanium materials have a high resistance to corrosion in aqueous hydrochloric acid, sulfuric acid and nitric acid. Thus, Taki, et al. teach that one should form a layer of titanium nitride, titanium carbide, or titanium carbonitride on a titanium surface to provide an anticorrosive effect.
On the other hand, among aqueous solutions, little is known about hydrogen absorption by titanium in aqueous hydrogen sulfide solutions. The only literature available is the publication "CORROSION", Vol 35, No 8(1979), pp. 378-382. This publication, however, deals principally with the hydrogen absorption by titanium in contact with dissimilar metals in aqueous hydrogen sulfide solutions. It is totally silent as to the prevention of hydrogen absorption by titanium, when titanium is the only metal in contact with the aqueous hydrogen sulfide solution.
Oil refining plants must remove sulfur from crude oil and, to achieve the end, they add hydrogen to the oil and separate the sulfur in the form of hydrogen sulfide. The separated hydrogen sulfide flows through piping and the tubing of heat exchangers as an aqueous solution. We have found that when pure titanium alone, out of contact with any dissimilar metal, is immersed in such a solution, it undergoes vigorous hydrogen absorption. Solutions that contain hydrogen sulfide are generally so corrosive that ordinary metallic materials, such as copper alloys and stainless steels, are unable to withstand their attacks. One way of coping with these environments has been to use titanium that presents no corrosion problem, albeit there is the possibility of hydrogen absorption. Another alternative is to depend on much less expensive carbon steel for short-term service, with repeated replacements.